A Multiple-Input Multiple-Output (MIMO) system refers to a system with a plurality of antennas equipped at both a transmitter and a receiver. A process in the space field is introduced to the MIMO system in addition to traditional processes in the time and frequency domains to thereby further obtain an array process gain and a diversity gain. In the MIMO system, the transmitter can optimize a transmission signal according to a channel characteristic if it can obtain channel information somehow to thereby improve a reception quality and lower required complexity of the receiver. Linear pre-coding/beam-forming is such an optimization method as effective means to deal with a fading channel, to lower an error probability and to improve the performance of the system.
For multi-antenna linear pre-coding/beam-forming transmission, information of a channel from a base station to a User Equipment (UE) is an important factor influencing the performance of a system. In a Frequency Division Duplex (FDD) system, the UE feeds the estimated channel information back to the base station over an uplink channel, and this scheme occupies considerable resources of the uplink channel and may introduce a quantization error, etc. In a Time Division Duplex (TDD) system, uplink and downlink signals are transmitted over the same frequency band, and thus reciprocity of uplink and downlink channels holds. The so-called reciprocity refers to that an uplink channel is identical to a downlink channel. With the reciprocity of the uplink and downlink channels, the uplink channel can be estimated from the uplink signal transmitted from the UE to thereby obtain downlink channel information while dispensing with considerable feedback overhead.
Channel reciprocity holds to a spatially propagating physical channel. A signal will be fed to an antenna through a transmission circuit after being subjected to a baseband process, and a signal received from an antenna will also be fed to a baseband through a reception circuit. The transmission circuit refers to a circuit through which the signal goes from a baseband processing unit to an input port of the antenna, and the reception circuit refers to a circuit through which the signal goes from an output port of the antenna to the baseband processing unit. Generally the transmission circuit and the reception circuit are two different circuits, so the transmission circuit and the reception circuit introduce different delays and amplitude gains, in other words, the transmission and reception circuits are mismatched. Reciprocity of uplink and downlink channels may not exactly holds due to the mismatch of the transmission circuit and the reception circuit. Specifically, this is denoted as:HkDL(f)=αk(f)e−j2πΔkfHks(f)η(f)e−j2πΩf  (1).
HkDL(f) is an equivalent downlink baseband channel from the kth antenna of the base station to the antenna of the UE (given here only one reception antenna of the UE), which includes a spatially propagating channel Hks(f), an amplitude response αk(f) and a phase response e−j2πΔkf of a transmission circuit of the kth antenna of the base station, and an amplitude response η(f) and a phase response e−j2πΩf of a reception circuit of the antenna of the UE, where Δk and Ω are delays caused by the transmission circuit of the kth antenna of the base station and the reception circuit of the antenna of the UE respectively, and f is a frequency.
An equivalent uplink baseband channel from the antenna of the UE to the kth antenna of the base station is denoted as:HkUL(f)=βk(f)e−j2πΠkfHks(f)ω(f)e−j2πΨf  (2).
It includes an amplitude response βk(f) and a phase response e−j2πΠkf of a reception circuit of the kth antenna of the base station, and an amplitude response ω(f) and a phase response e−j2πΨf of a transmission circuit of the antenna of the UE, where Ψ and Πk are delays caused by the transmission circuit of the antenna of the UE and the reception circuit of the kth antenna of the base station respectively.
As can be apparent from comparison of Equation (1) with Equation (2), the equivalent uplink baseband channel and the equivalent downlink baseband channel may differ from each other even with the same spatially propagating channel.
An equivalent uplink baseband channel from the antenna of the UE to a number M of antennas of the base station is written in the form of a vector as:
                                          H            UL                    ⁡                      (            f            )                          =                ⁢                  [                                                    H                1                UL                            ⁡                              (                f                )                                      ,            …            ⁢                                                  ,                                          H                M                UL                            ⁡                              (                f                )                                              ]                                        =                ⁢                              ω            ⁡                          (              f              )                                ⁢                                    ⅇ                                                -                  j                                ⁢                                                                  ⁢                2                ⁢                π                ⁢                                                                  ⁢                Ψ                ⁢                                                                  ⁢                f                                      ⁡                          [                                                                    H                    1                    s                                    ⁡                                      (                    f                    )                                                  ,                …                ⁢                                                                  ,                                                      H                    M                    s                                    ⁡                                      (                    f                    )                                                              ]                                                                      ⁢                              [                                                                                                                              β                        1                                            ⁡                                              (                        f                        )                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  Π                          1                                                ⁢                        f                                                                                                              0                                                  0                                                                              0                                                  ⋱                                                  0                                                                              0                                                  0                                                                                                                    β                        M                                            ⁡                                              (                        f                        )                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  Π                          M                                                ⁢                        f                                                                                                                  ]                    .                    
An equivalent downlink baseband channel from the M antennas of the base station to the antenna of the UE is written in the form of a vector as:
                                          H            DL                    ⁡                      (            f            )                          =                ⁢                  [                                                    H                1                DL                            ⁡                              (                f                )                                      ⁣                          ,              …              ⁢                                                          ,                                                H                  M                  DL                                ⁡                                  (                  f                  )                                                              ]                                        =                ⁢                              η            ⁡                          (              f              )                                ⁢                                    ⅇ                                                -                  j                                ⁢                                                                  ⁢                2                ⁢                π                ⁢                                                                  ⁢                Ω                ⁢                                                                  ⁢                f                                      ⁡                          [                                                                    H                    1                    s                                    ⁡                                      (                    f                    )                                                  ,                …                ⁢                                                                  ,                                                      H                    M                    s                                    ⁡                                      (                    f                    )                                                              ]                                                                      ⁢                              [                                                                                                                              α                        1                                            ⁡                                              (                        f                        )                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  Δ                          1                                                ⁢                        f                                                                                                              0                                                  0                                                                              0                                                  ⋱                                                  0                                                                              0                                                  0                                                                                                                    α                        M                                            ⁡                                              (                        f                        )                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  Δ                          M                                                ⁢                        f                                                                                                                  ]                    .                    
Given a downlink transmission scheme of Maximum Ratio Transmission (MRT), a downlink pre-coding vector (beam-forming weighting factor) is calculated from HUL(f) as:
            w      ⁡              (        f        )              =                            (                                    H              UL                        ⁡                          (              f              )                                )                H                                                  H            UL                    ⁡                      (            f            )                                        ;
where (A)H is a complex conjugate transpose of a vector A, and ∥A∥ is the norm of the vector A.
A signal received by the UE is represented as:
                    r        =                ⁢                                                            H                DL                            ⁡                              (                f                )                                      ⁢                          w              ⁡                              (                f                )                                      ⁢                          s              ⁡                              (                f                )                                              +                      n            ⁡                          (              f              )                                                              =                ⁢                                                            H                DL                            ⁡                              (                f                )                                      ⁢                                                  ⁢                                                            (                                                            H                      UL                                        ⁡                                          (                      f                      )                                                        )                                H                                                                                                  H                    UL                                    ⁡                                      (                    f                    )                                                                                        ⁢                          s              ⁡                              (                f                )                                              +                      n            ⁡                          (              f              )                                                              =                ⁢                                                                              (                                                            η                      ⁡                                              (                        f                        )                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                        Ω                        ⁢                                                                                                  ⁢                        f                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                        Ψ                        ⁢                                                                                                  ⁢                        f                                                                              )                                                                                                                          ∑                                          k                      =                      1                                        M                                    ⁢                                                                                    α                        k                                            ⁡                                              (                        f                        )                                                              ⁢                                          ⅇ                                                                        -                          j                                                ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  Δ                          k                                                ⁢                        f                                                              ⁢                                                                  β                        k                                            ⁡                                              (                        f                        )                                                              ⁢                                          ⅇ                                              j                        ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  Π                          1                                                ⁢                        f                                                              ⁢                                                                                                                                                H                            k                            s                                                    ⁡                                                      (                            f                            )                                                                                                                      2                                                                                                                                              ∑                                  k                  =                  1                                M                            ⁢                                                                                                                                      β                        k                                            ⁢                                                                                          ⁢                                              (                        f                        )                                                                                                  2                                ⁢                                                                                                                        H                        k                        s                                            ⁡                                              (                        f                        )                                                                                                  2                                                                                                    ⁢                                            s              ⁡                              (                f                )                                      +                          n              ⁡                              (                f                )                                              ;                                    where s(f) and n(f) are a data symbol transmitted to the UE and an additive noise respectively. If reciprocity of the uplink and downlink channels exactly holds, that is, αk(f)=βk(f) and Δk=Πk, w(f) is such that signals of the respective antennas received by the UE are superimposed in phase, and at this time the UE receives a signal with the highest signal to noise ratio. If the uplink and downlink circuits are mismatched and particularly have different delays, in-phase superimposition of the signals of the respective antennas cannot be ensured, thus lowering the signal to noise ratio of the received signal and deteriorating the performance.        
A method for reducing the influence resulting from mismatched uplink and downlink circuits is to perform antenna calibration.
A calibration factor is calculated from information reported from the UE and/or information measured at the base station, and a channel estimated from an uplink signal is compensated and adjusted, or data to be transmitted is compensated and adjusted.
Two common antenna calibration schemes in the prior art will be introduced below.
Calibration Scheme 1:
1. The UE estimates delay values introduced by radio circuit lines and transmission filters (collectively referred below to as transmission circuits) of respective transmission antennas. The UE firstly estimates an equivalent downlink baseband channel from a transmission antenna to the UE at a specific frequency f, which includes a response of a transmission circuit, a spatially propagating channel and a response of a reception circuit. A phase of an equivalent downlink baseband channel from the kth transmission antenna to the UE is denoted as Θk(f) which can be represented as:Θk(f)=φh,k(f)+2πΔkf; 
where the first term represents a phase of the equivalent downlink baseband channel of both the propagating channel and the response of the reception circuit of the UE, and Δk is a delay introduced by the transmission circuit of the kth transmission antenna of the base station. Without loss of generality, the UE can estimate the difference in time Δk−Δ1 between the kth and 1st antennas, particularly by linearly fitting Θk(f)−Θ1(f) so that the slope of a resulting straight line is Δk−Δ1:Θk(f)−Θ1(f)=φh,k(f)−φh,1(f)+2π(Δk−Δ1)f. 
2. The UE quantizes and then feeds the obtained Δk−Δ1 back to the base station.
3. The UE transmits a Sounding Reference Signal (SRS) or another uplink signal, and the base station estimates channels from the antenna of the UE to the respective reception antennas according to the SRS or the another uplink signal and estimates a delay Πk−Π1 introduced by the reception antenna circuit of the base station as in the step 1.
4. The base station compensates the uplink channel estimated by the base station according to Δk−Δ1 reported from the UE and the estimated Πk−Π1. Given the uplink channel HkUL(f) estimated by the base station, the estimated value of a compensated downlink channel is:HkDL(f)=HkUL(f)e−j2π(Δk−Δ1)fe−jπ(Πk−Π1)f.
Calibration Scheme 2.
1. The UE firstly estimates an equivalent downlink baseband channel from a transmission antenna to the UE at a specific frequency f, which includes a response of a transmission circuit, a spatially propagating channel and a response of a reception circuit. An equivalent downlink baseband channel from the kth transmission antenna to the UE is denoted as HkDL(f).
2. The UE quantizes and then feeds the obtained HkDL(f) back to the base station.
3. The UE transmits an SRS or another uplink signal, and the base station estimates channels HkUL(f) from the antenna of the UE to the respective reception antennas according to the SRS or the another uplink signal.
4. The base station calculates a calibration coefficient from HkDL(f) reported from the UE and the estimated HkUL(f):c′k=HkDL(f)/HkUL(f).
Without loss of generality, all the calibration coefficients can be normalized by the calibration coefficient of the antenna 1:ck=c′k/c′1.
5. The base station compensates the estimated uplink channel by the obtained calibration coefficient to obtain a downlink channel as:HkDL(f)=HkUL(f)ck.
Drawbacks of the prior art lie in as follows: the problem of the calibration scheme 1 lies in that only a phase error but no amplitude error can be calibrated, and an amplitude error may also have a serious influence on the performance; and the calibration scheme 2 can calibrate both a phase error and an amplitude error, but this scheme feeds a quantized equivalent downlink baseband channel back directly to the base station, and an estimation error and a quantization error of the equivalent downlink baseband channel may degrade the precision of calibration. Furthermore, if the UE has a channel varying with time, a spatially propagating channel measured at the UE may have varied from that measured at the base station, thus causing inaccurate calculation of a calibration coefficient.